New usePrefix settings option

Thanks to @rayriffy you can configure an array of prefix options #47.

usePrefix: ["embed", "video", "oembed"]

// In your markdown file

Check it out! I can use the prefix "oembed:"

`oembed: https://twitter.com/raae/status/1045394833001652225`

Or I can use the prefix "embed:" if I like ;)

`embed: https://www.instagram.com/p/Bof9WhgBmY2/`

I can also use "video:" like I did before with `gatsby-remark-video`.

`video: https://vimeo.com/42672205`

In addition:

• @raae fixed a logging bug #73
• @testower made sure we have a copy of the oembed provider list in case oembed.com goes down #64
• @raae, @rayriffy, @muescha, and @yurm04 squashed some maintenance tasks